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Investigators suspect Joshua Petersen had been planning to kill his baby son and himself for about a month, an American Fork police sergeant said Saturday.

Petersen has been cooperative with investigators since his arrest, though they're still trying to piece together his motive, Sgt. Greg Ludlow said. Peter­sen's family, some of whom had been living with him at the home, is "a very good one," he added.

Police say Peter­sen, 21, shot his 5-month-old son, Ryker Petersen in the head with a rifle on Friday afternoon at his home near 500 East and 500 North in American Fork, and a relative stopped him before he turned the gun on himself. Ryker was pronounced dead at a Salt Lake City hospital, and Petersen was booked into Utah County Jail on suspicion of aggravated murder, a first-degree felony.

Ryker's mother, Amanda Merrill Pilling, is devastated by the loss of her young son, her family said in a statement issued late Saturday.

"Her sweet baby Ryker was a treasure and she misses him immensely," the statement provided by family spokesman Milo Merrill said. "Our hearts go out to all of you who are suffering for the loss of baby Ryker, including the father's family. We cannot imagine how hard this must be for you and we send our love and support to you."

Pilling's family said they understand how it is "easy to jump to conclusions and judgment when a tragedy like this occurs," but said they believed all judgments should be left to God. The family also expressed their gratitude to those who have reached out to support them in their grief.

"We will not let this sadness consume us," the family said.

Petersen struggled with depression his whole life, but Ryker had been a source of happiness, Petersen's aunt Staci Zufelt said.

"He had trouble in school. He was just kind of a loner," Zufelt said. Then in late October, he became a father. Zufelt said when she saw Petersen, he usually had Ryker with him. She said he was happy around his son.

"He really loved his son," who was a "very happy boy," Zufelt said.

Petersen only saw Ryker a few hours a day, Zufelt said. Otherwise, the baby was with Pilling. Zufelt declined to comment on Petersen and Pilling's relationship. Posts on Petersen's Facebook account include pictures of his young son. In the weeks prior to the shooting, Petersen also posted multiple comments indicating he had been having a "bad day" but that being with his son made it better.

Conversely, some posts indicated that his mood also was affected negatively when he wasn't with Ryker.

"Not cool, I don't have my son," reads one post from Feb. 27

Zufelt had no idea if Peter­sen had contemplated suicide before.

Petersen has no criminal history, according to a search of Utah court records. Depending on the severity of the murder charge filed against Petersen and aggravating factors, Petersen could face anything from life in prison to the death penalty.

Pilling's family has planned a candlelight vigil for Ryker at 7 p.m. Sunday at 36 S. Jenks Ave. (140 West) in American Fork. Attendees are encouraged to bring candles and toys.

A Paypal account has also been set up to help the family give Ryker "an amazing funeral," according to a Facebook page about the vigil. Donations at Paypal are to be sent to jkfifield@gmail.com with "Baby Ryker" in the subject line.

Petersen is scheduled to go before a judge Monday to set his bail, Ludlow said.